In a conventional gas turbine; a compressor pressurizes air, which is channeled to a combustor, mixed with fuel, and ignited for generating combustion gases which flow to a turbine disposed downstream therefrom. The turbine extracts energy from the hot combustion gases for driving the compressor and a generator.
Conventional turbines include one or more stages of stationary vanes and rotating blades, which typically extract energy from the combustion gases by reaction. The combustor, blades and vanes are typically air cooled by a portion of the air pressurized by the compressor in order to provide acceptable life in the gas turbine engine. However, any portion of the compressed air that is utilized for cooling the hot gas path parts (combustor, blades, vanes etc.) is not available for undergoing combustion, which reduces the overall efficiency and power of the engine. Accordingly, it is desirable to use as little of the compressed air as possible in cooling the hot gas path parts, in particular the blades and vanes, consistent with obtaining relatively long useful life of the hot gas parts, which is typically accomplished by providing heat transfer enhancement members such as elongate turbulator ribs on the cooling side of the hot gas path parts.
Gas turbine engine hot gas path parts such as for example an airfoil portion over which the combustion gases flow typically comprises cooled sidewalls with an internal passage for channeling the cooling air. The term hot gas path part used herein shall mean any member placed within the gas turbine engine flowpath over which the hot combustion gases flow, such as burner walls, combustor walls or liners as well as rotor blades or stator vanes. Rotor blades or stator vanes are simply referred to as blades in the following description.
Turbulator ribs typically used in cooled airfoils are conventionally formed as part of the blade casting and project inwardly into the internal cooling passages of the blade through which the cooling air is channeled. The ribs enhance the convective heat transfer coefficient along the inner surface of the blade by tripping or disrupting the cooling air boundary layer, which is caused to separate from the internal surface and then reattach downstream from the rib. The heat transfer coefficient enhancement is conventionally defined as the convective heat transfer coefficient effected by the ribs divided by the convective heat transfer coefficient over a smooth surface without turbulator ribs, and has values ranging up to several times that of the latter. Typically also the ribs of other hot gas path parts are integrally formed as part of the part, e.g. during casting.
Enhancement is conventionally related to the height or projection of the ribs into the internal passage, the distance between opposing walls of the internal passage, and the distance or spacing longitudinally between the ribs. Exemplary turbulator ribs may include ribs disposed perpendicularly to the direction of cooling flow, ribs inclined relative to the direction of the cooling airflow, and ribs disposed on opposite walls of the internal passage that are longitudinally positioned either in-line or staggered with respect to each other.
Turbulator ribs provide localized increases in enhancement, which decrease rapidly in value downstream from each individual rib. For obtaining a generally uniform cooling enhancement along the surface of the wall cooled the ribs are typically uniform in configuration, uniform in height or projection into the internal passage, and uniform in longitudinal spacing.
The various conventional turbulator ribs result in different amounts of enhancement, along with pressure losses associated therewith. Since the ribs project into the internal passage and partially obstruct the free flow of the cooling air therethrough, they provide resistance to the flow of the cooling air, which results in pressure losses. Although higher ribs generally increase enhancement, the pressure drop associated therewith also increases. Accordingly, the effectiveness of turbulator ribs must be evaluated by their ability to provide effective enhancement without undesirable levels of pressure losses associated therewith.
A cut of cooled wall 1, 2 with an ideal conventional turbulator rib for cooling enhancement is shown in FIG. 2. The cooled wall 1, 2 has a wall thickness t. It has a smooth surface on the hot side 15 and turbulator ribs 5 having a height h and a width w extend into the cooling flow 14. The ideal turbulator ribs have acute corners at their tips and roots for good heat transfer enhancement. They are spaced apart with a pitch p.
Even so the ideal turbulator rib of FIG. 2 leads to a good heat transfer enhancement their shape can typically not be realized for practical reasons. Manufacturing of acute angles typically requires machining. However, most of the hot gas parts of a gas turbine are cast, and therefore require minimum radii. Further, hot gas parts are typically coated. Coating material tends to smoothen sharp corners and increase the radius of any curved shape. In reality a cooled wall 1, 2 with turbulator ribs 5 deviates from the ideal shape shown in FIG. 2, and its turbulator ribs 5 have rounded corners. FIG. 3 shows a cut of a cooled wall 1, 2 with realistic conventional turbulator ribs 1, 2 for cooling enhancement. The roots of the turbulators 5 are formed with fillets having a radius R1, and the tips are rounded with a radius R2.